The present invention relates to a method for separating entrained particles from a gas in a fluidised bed reactor system and a fluidised bed reactor system including a particle separator for separating entrained particles from a gas.
In the fields of pyrolysis, gasification and combustion, it is common to provide the reactor of a boiler or a combustion apparatus with a bed of particles, which, among other advantages, greatly enhances heat transfer because of the high heat carrying capacity of the particles. The bed is usually placed in the lower portion of the reactor. Fluidising air or gas entrains the particles with a gas flow inside the reactor. At the upper portion of the reactor, or outside the reactors the particles are separated from the gas flow by separators. In a circulating fluidised bed the particles are recirculated to the lower portion of the reactor, from where they can once again be entrained in the gas flow.
There are basically two types of separators: non-centrifugal mechanical particle separators and cyclone-type particle separators.
Examples of non-centrifugal mechanical particle separators are disclosed in WO 83/03294, U.S. Pat. No. 5,025,755, U.S. Pat. No. 5,082,477 and U.S. Pat. No. 5,064,621.
In WO 83/03294 a boiler is disclosed having a non-centrifugal mechanical particle separator outside the reactor.
In U.S. Pat. No. 5,025,755 an apparatus is disclosed having a non-centrifugal mechanical particle separator in the upper portion of the reactor.
An example of a cyclone-type particle separator disposed in the upper portion of a reactor is disclosed in U.S. Pat. No. 5,070,822.
An object of the present invention is to achieve a compact particle separator.
Another object of the invention is to achieve a particle separator that is easily mountable and demountable inside a reactor.
These and other objects which will become apparent in the following are achieved by a fluidised bed reactor system and a method for separating particles as defined in the accompanied claims.
The present invention is based on the insight of the advantages of separating particles in a direction other than the xe2x80x9cmain flow directionxe2x80x9d. The term xe2x80x9cmain flow directionxe2x80x9d is generally referred to here as the direction of a line drawn between a point before the gas enters the separator and a point after the gas exits the separator. In prior art non-centrifugal mechanical separators, the separator elements are conventionally positioned so as to separate the particles from the gas flowing substantially in the xe2x80x9cmain flow directionxe2x80x9d. In other words, the separation direction is one-dimensional. According to the present invention, however, the particles can be separated from the gas flowing in a direction other than the xe2x80x9cmain flow directionxe2x80x9d, whereby the separation is multidimensional.
Also, it has been realised that the particle separator can be made compact in a configuration that allows the gas to pass from the outside of the configuration to the inside thereof and/or vice versa, wherein the particles are separated from the gas during such a travel.
According to one aspect of the present invention a method is provided for separating entrained particles from a gas in a fluidised bed reactor system which comprises a separation region defined by a cylindrical r-,xcfx86-,z-coordinate system, the method comprising the consecutive steps of:
leading the gas in the z-direction (axial direction),
diverting the gas to flow substantially in the r-direction (radial direction), while keeping the gas circumferentially distributed in rxcfx86-planes, and
mechanically separating the particles from the gas while the gas is flowing substantially in the r-direction.
According to another aspect the present invention provides a fluidised bed reactor system including a particle separator for separating entrained particles from a gas having a flow path. The particle separator comprises a set of non-centrifugal mechanical separator elements disposed in the flow path of the gas, so that the gas is able to pass between the separator elements while the inertia of the particles directs them to the separator elements upon which they impinge and are separated and removed from the gas flow. The set of separator elements is arranged in a configuration having a centre zone with a centre axis, and a circumference. Directional means are provided for directing the gas so that gas passing through the set of separator elements flows from the circumference to the centre zone of the configuration and/or vice versa.
Thus, as mentioned above, according to the present invention the particles are separated from the gas multidimensionally instead of the traditional one-dimensional separator passage as far as non-centrifugal mechanical particle separator elements are concerned. In mathematical terms, instead of a separation in the x-direction in an orthonormal x-,y-,z-coordinate system, the present invention provides separation in the r-direction in a cylindrical r-,xcfx86-,z-coordinate system, where:
xe2x80x83r=xxc2x7cos xcfx86+yxc2x7sin xcfx86
Hence, the region where the separation is performed is conveniently defined by a cylindrical coordinate system. Gas will be led to flow in the z-direction or axial direction of the separation region. Thereafter, the gas is diverted to flow substantially in the re-direction or radial direction of the separation region. This does not necessarily mean that the gas will be diverted in a direction which is totally perpendicular to the axial direction, but merely that the gas will flow to or from a centre zone. During this diverting action the gas is kept circumferentially distributed in rxcfx86-planes, i.e. disk shaped planes. Accordingly, the gas does not have to flow from or to just one side of the separation region, but substantially from or to the whole circumference of the separation region. It is during this radial flow that the particles are separated from the gas.
According to a further aspect the separator elements are arranged as a structure having consecutive particle separation levels XN (X1,X2,X3, . . . , Xn . . . ), N being an integer. The directional means are arranged at the circumference and at the centre zone of the configuration, so as to cause the gas to flow through the separator elements in one direction on levels with odd-numbered N and in the reversed direction on levels with even-numbered N.
The obvious advantage of this is that, when the separator elements preferably being provided as one set of separator elements, one and the same separator element is passed by the flowing gas repeated times. Thus particles that have not impinged upon the separator at the first pass, can be captured on the following pass(es), thus making the most of each separator element.
Aptly, the configuration has a generally cylindrical shape, preferably with the separator elements being arranged essentially symmetrically. Note that the term xe2x80x9ccylindricalxe2x80x9d does not necessarily imply that the cross-section is circular.
Preferably, the separator elements have an elongated shape and extend essentially in parallel with the centre axis.
It is advantageous to use channel-shaped beams as separator elements, the beams having an essentially U-shaped cross-section. The beams are arranged so that the particles impinge upon the bottom of the U and then fall down, guided by the channel-shaped beam, to be collected.
In order to further enhance the efficiency of the system, the set of separator elements can form a number of ring-shaped arrays being placed within each other. The separator elements of an array are preferably circumferentially displaced with respect to the separator elements of an adjacent array.
Consequently, the separator elements of the different arrays can be arranged in a staggered way with an angular offset with respect to each other. Those particles that do not impinge on separator elements of one array can be disentrained from the gas to a great extent by the separator elements of an adjacent array. Of course the number of arrays is chosen according to what is considered appropriate, with respect to compactness, efficiency etc.
According to a specific embodiment, each separator element, being in form of a U-shaped beam, is provided with a respective additional U-shaped beam attached in parallel thereto. Moreover, each of the additional U-shaped beams is provided with a respective further U-shaped beam attached in parallel thereto, forming a unit with three U-shaped beam channels. Dividing plates are inserted in at least two U-shaped beam channels for mechanical segregation of said channels and a section of at least one of the elements in the unit is removed, so as to create three particle separation levels of impinge areas, one for each element in the unit. Directional means are arranged to direct the gas in alternating level directions.
A three-channel unit design can be constructed with three identical U-beams or with three non-identical U-beams. For instance, a tapered design may be used. This is particularly practical inside a circular reactor shaft, in which case the element located nearest the shaft centre would have a smaller cross-section than the intermediate element, which in turn would have a smaller cross-section than the element furthest away from the centre.
Due to the configuration of a particle separator according to the invention, it is particularly suitable for disposal inside a reactor shaft. Even though the separator elements are preferably arranged in a symmetrical and circular configuration, it is also possible to arrange the separator elements in other configurations, such as triangular, square, other polygon or in any other desired way. When the particle separator is intended to be used inside a reactor shaft, it is favourable to have the configuration adapted to the cross-section of the reactor shaft.
After the particles have been disentrained they fall down from the separator elements to some form of collector located below. The disentrained particles can advantageously be recycled to the reactor bed by a standpipe.
When the particle separator is disposed inside the reactor, an internal standpipe located around the centre axis of the reactor can be used. In this case, the fluidising gas with entrained particles suitably flows from the bottom portion to the top portion of the reactor, generally symmetrically around the internal standpipe. The particle separator, preferably being disposed at the top portion of the reactor, disentrains the particles from the gas, which exits the reactor. The particles are then recycled through the internal standpipe in the centre of the reactor.
Of course it is also possible to let an internal standpipe be positioned off-centred, e.g. extending along the wall of the reactor. In this case it might be desirable to have more than one standpipe. The choice of an off-centred alternative provides for the possibility of letting the gas enter the particle separator from the centre of the configuration and consequently the disentrained particle can advantageously be caused to fall down at the circumference thereof.
The above description is related to a circulating fluidised bed. The person skilled in the art will realise that the present invention can be utilised in other connections as well. The skilled person will also realise that the particle separator of the present system can be located outside a reactor, and not only inside.